使用makecontext实现用户线程       现代Unix系统都在ucontext.h中提供用于上下文切换的函数，这些函数有getcontext, setcontext，swapcontext 和makecontext。其中，getcontext用于保存当前上下文，setcontext用于切换上下文，swapcontext会保存当前上下文并切换到另一个上下文，makecontext创建一个新的上下文。实现用户线程的过程是：我们首先调用getcontext获得当前上下文，然后修改ucontext_t指定新的上下文。同样的，我们需要开辟栈空间，但是这次实现的线程库要涉及栈生长的方向。然后我们调用makecontext切换上下文，并指定用户线程中要执行的函数。       在这种实现中还有一个挑战，即一个线程必须可以主动让出CPU给其它线程。swapcontext函数可以完成这个任务，图4展示了一个这种实现的样例程序，child线程和parent线程不断切换以达到多线程的效果。在libfiber-uc.c文件中可以看到完整的实现。#include #include #include // 64kB stack#define FIBER_STACK 1024*64ucontext_t child, parent;// The child thread will execute this functionvoid threadFunction(){printf( "Child fiber yielding to parent" );swapcontext( &child, &parent );printf( "Child thread exiting\n" );swapcontext( &child, &parent );}int main(){// Get the current execution contextgetcontext( &child );// Modify the context to a new stackchild.uc_link = 0;child.uc_stack.ss_sp = malloc( FIBER_STACK );child.uc_stack.ss_size = FIBER_STACK;child.uc_stack.ss_flags = 0; if ( child.uc_stack.ss_sp == 0 ){perror( "malloc: Could not allocate stack" );exit( 1 );}// Create the new contextprintf( "Creating child fiber\n" );makecontext( &child, &threadFunction, 0 );// Execute the child contextprintf( "Switching to child fiber\n" );swapcontext( &parent, &child );printf( "Switching to child fiber again\n" );swapcontext( &parent, &child );// Free the stackfree( child.uc_stack.ss_sp );printf( "Child fiber returned and stack freed\n" );return 0;}图4用makecontext实现线程用户级线程的抢占       抢占实现的一个最重要的因素就是定时触发的计时器中断，它的存在使得我们能够中断当前程序的执行，异步对进程的时间片消耗情况进行统计，并在必要的时候（可能是时间片耗尽，也可能是一个高优先级的程序就绪）从当前进程调度到其它进程去执行。       对于用户空间程序来说，与内核空间的中断相对应的就是信号，它和中断一样都是异步触发，并能引起执行流的跳转。所以要想实现用户级线程的抢占，我们可以借助定时器信号(SIGALRM)，必要时在信号处理程序内部进行上下文的切换。       为了验证自己的想法，我在上篇文章提到的协同多线程的基础上加上了相关抢占代码，具体实现如下：#include <stdlib.h>#include <stdio.h>#include <ucontext.h>#include <sys/time.h>#define STACK_SIZE 4096#define UTHREAD_MAX_NUM 256#define INIT_TICKS 10typedef int uthread_t;typedef void uthread_attr_t;uthread_t current = 0;#define uthread_self() currentstruct uthread_struct{        int used;        ucontext_t context;        char stack[STACK_SIZE];        void* (*func)(void *arg);        void *arg;        void *exit_status;        int ticks;};static struct uthread_struct uthread_slots[UTHREAD_MAX_NUM];void panic(void){                fprintf(stderr, "Panic, bala bala...\n");                exit(EXIT_FAILURE);}void idle_thread(void){        int i;        for (i = 1; i < UTHREAD_MAX_NUM; i ++)                if (uthread_slots[i].used)                        break;        if (i == UTHREAD_MAX_NUM)                panic();        if (current != 0)                uthread_slots[current].used = 0;        current = i;        swapcontext(&uthread_slots[0].context,&uthread_slots[current].context);}void uthread_context_init(int tid){        getcontext(&uthread_slots[tid].context);        uthread_slots[tid].context.uc_stack.ss_sp = uthread_slots[tid].stack;        uthread_slots[tid].context.uc_stack.ss_size =sizeof(uthread_slots[tid].stack);        uthread_slots[tid].context.uc_link = &uthread_slots[0].context;}void uthread_init(void){        uthread_context_init(0);        uthread_slots[0].used = 1;        makecontext(&uthread_slots[0].context, idle_thread, 0);}void uthread_schedule(void);void uthread_exit(void *exit_status){        uthread_slots[current].exit_status = exit_status;        uthread_slots[current].used = 0;        uthread_schedule();}void uthread_helper(void){        uthread_exit(uthread_slots[current].func(uthread_slots[current].arg));}int uthread_create(uthread_t *thread, const uthread_attr_t *attr,                        void* (*start_routine)(void*), void *arg){        static int last_used = 0;        int i;        for (i = (last_used + 1) % UTHREAD_MAX_NUM; i != last_used;                        i = (i + 1) % UTHREAD_MAX_NUM)                if (!uthread_slots[i].used)                        break;        if (i == last_used)                return -1;        last_used = i;        if (thread != NULL)                *thread = i;        uthread_context_init(i);        uthread_slots[i].used = 1;        uthread_slots[i].func = start_routine;        uthread_slots[i].arg = arg;        uthread_slots[i].exit_status = 0;        uthread_slots[i].ticks = uthread_slots[current].ticks / 2;        uthread_slots[current].ticks -= uthread_slots[i].ticks;        makecontext(&uthread_slots[i].context, uthread_helper, 0);        return 0;}void uthread_schedule(void){        int i, prev;        for (i = (current + 1) % UTHREAD_MAX_NUM; i != current;                        i = (i + 1) % UTHREAD_MAX_NUM)                if (uthread_slots[i].used)                        break;        if (i == current)                panic();        prev = current;        current = i;        swapcontext(&uthread_slots[prev].context,&uthread_slots[current].context);}void* thread(void *arg){        int i;        for (i = 0; 1; i ++) {                if (i % 1000 == 0)                        printf("thread/%d(%s): i = %d\n", current, (char*)arg,i);                uthread_create(NULL, NULL, thread, arg);                if (i % 1000000 == 0)                uthread_schedule();        }}void sig_ticks_timer(int signo){        if (--uthread_slots[current].ticks <= 0) {                uthread_slots[current].ticks = INIT_TICKS;                uthread_schedule();        }}int main(int argc, char *argv[]){        uthread_t tid;        struct itimerval ticks_timer;        uthread_init();        uthread_create(&tid, NULL, thread, "hw1");        printf("tid is %d\n", tid);        uthread_create(&tid, NULL, thread, "hw2");        printf("tid is %d\n", tid);        signal(SIGALRM, sig_ticks_timer);        ticks_timer.it_interval.tv_sec = 0;        ticks_timer.it_interval.tv_usec = 10000;        ticks_timer.it_value.tv_sec = 0;        ticks_timer.it_value.tv_usec = 10000;        setitimer(ITIMER_REAL, &ticks_timer, NULL);        while (1)                idle_thread();        return 0;}       似乎该有的都有了，也许真的可以在用户空间实现一个虚拟的操作系统环境玩呢...